Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, e.g., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, an isolation or access device, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and a structural state of generally crystalline local order or between detectable structural states of differing local order across the entire spectrum between completely amorphous and completely crystalline states. These different structural states have different values of resistivity and therefore, each state can be determined or distinguished by electrical sensing. Typical materials suitable for such applications include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
The phase-change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, as that value represents a physical state of the material (e.g., crystalline, amorphous, or partially crystalline/partially amorphous). Furthermore, reprogramming requires little energy to be provided and dissipated in the device. Thus, phase-change memory materials represent a significant improvement in non-volatile memory technology.
In an array of phase-change memory devices, it is necessary to be able to read and write individual devices without disturbing neighboring devices. In most practical arrays, the memory devices are interconnected between word lines and bit lines. The word lines and bit lines permit addressing of the memory devices. Each word line is connected to several memory devices (e.g. multiple devices in a row of an array). Each bit line is also connected to several memory devices (e.g. devices in a column of an array). Each memory device, however, is interconnected between a single word line and a single bit line, where the word line-bit line combination is unique for each memory device. This configuration permits each memory device to be individually addressed or selected for reading and writing.
To read and write, a memory device is selected by applying a voltage between the word line and bit line to which the device is interconnected. Since each word line is connected to other memory devices in the row of the selected device and since each bit line is connected to other memory devices in the column of the selected device, read and write voltages applied to the word line and bit line of the selected memory device create ancillary voltage changes that may influence the structural state of non-selected devices. To prevent ancillary voltages from disturbing the structural state of non-selected devices, it is common to include an isolation device with each memory device in an array. The isolation device is normally interconnected between the word line and bit line that address the memory device and in series with the memory device. The purpose of the isolation device is to electrically isolate the memory device from residual voltages that exist within an array. When a memory device is not selected, the isolation device is in a high resistance OFF state that prevents leakage of current from the array to the non-selected memory device. When the memory device is selected, the voltage applied between the word line and bit line of the device is sufficient to transform the isolation device to a low resistance ON state that permits the currents needed for reading and writing to access the memory device.
The Ovonic Threshold Switch (“OTS”) is a promising isolation device. The OTS has an OFF state that is highly resistive and highly effective at preventing leakage currents from influencing non-selected devices in an array. In its ON state, the OTS is highly conductive and readily permits electrical interrogation of a memory device. The ON state of the OTS is produced when the voltage across the OTS exceeds a threshold value and is readily established by providing at least a threshold level voltage between the word line and bit line of a selected cell.
A drawback associated with the OTS, however, is that the transition from the OFF state to the ON state is accompanied by a snapback in voltage. When the threshold voltage is reached, the OTS voltage abruptly decreases and the current through the OTS abruptly increases. The snapback nature of OTS operation leads to variability in its operating characteristics that detract from practical performance. Variability can occur from cycle-to-cycle in the operation of a given OTS or as an inconsistency in operating parameters across different devices in an array. Variability in threshold voltage, for example, complicates the operation of a memory device because a given select voltage may switch the OTS to its ON state in one cycle and not in another cycle. Similarly, a particular threshold voltage may switch some OTS devices in an array and not others.
In addition to effective isolation of phase-change memory devices in an array, it is also desirable to operate the phase-change memory devices in an array with minimum power. The competitiveness of phase-change technology depends in part on achieving memory devices and arrays that can be programmed with low currents. To effect a transition of a phase-change material from one structural state to another, it is necessary to apply sufficient energy to heat the material. For electrical devices, the thermal energy is a consequence of Joule heating localized near the phase-change material that is produced by the programming current. To increase the local temperature for a given programming current, it is necessary to increase the resistance of the electrode or other circuit element adjacent to the phase-change material. It is further desirable to avoid interdiffusion of elements between the phase-change material and the locally heated electrode or circuit element adjacent to it. Common electrode materials such as TiN or TiAlN provide good protection against elemental interdiffusion, but are insufficiently resistive to permit programming at preferred current levels.
A need exists for a phase-change memory array that offers the low leakage currents of an OTS without the inconsistency of performance associated with OTS operation. The memory array should further offer low programming currents and a device configuration that prevents interdiffusion of elements between adjacent layers or devices.